1. Field of the Invention
This invention concerns a method and an apparatus for impact testing of bend specimens, and which reduce the inertial load effects in an impact test and also change the state of stress in the test specimen towards 3-point bending and therefore enable more reliable analysis of the data.
2. Description of the Prior Art
The impact test is generally used to measure the toughness of materials and the methods and the test specimens are standardized in most countries (ASTM E 370, DIN 51222). It is also a general practice to measure and to record the force which is acting on the specimen during the impact (Turner, E.C.: "Measurement of Fracture Toughness by Instrumented Impact Test", Impact Testing of Metals, ASTM STP 466, American Society for Testing and Materials, 1970, pp. 93-114; Instrumented Impact Testing, ASTM STP 563, American Society for Testing and Materials, 1974).
The impact test result is often presented in the form of a force-time or a force-displacement diagram. It is also general knowledge that when the hammer first hits the specimen, a significant force is observed because the mass of the specimen has to be accelerated to the speed of the hammer. This force is inevitable and it will occur even when the specimen is only supported and is free to fly away. It has been observed that the inertial load alone can break a brittle specimen (Radon, J.C. and Turner, C.E.: "Fracture Toughness Measurements by Instrumented Impact Test", Engng. Fracture Mech. Vol. 1, pp. 411-428, 1969).
This inertial force peak sets the specimen and the load transducer to a very rapid vibration and therefore the force measured from the instrumented tup bears no actual resemblance to the real force which is acting on the specimen during approximately the first 20 or 30 microseconds. Brittle specimens very often break during this initial period and in this case the inertial load effects completely mask the true breaking load and the true energy consumed in the fracture process (the consumed energy is usually calculated by integrating the area under the force-displacement diagram and it is almost impossible to separate the fracture energy from the energy consumed only for acceleration of the specimen).
The significance and the effects of the inertial load are presented in Saxton, H.J., Ireland, D.R. and Server, W.L.: "Analysis and Control of Inertial Effects During Instrumented Impact Testing," ASTM STP 563, American Society for Testing and Materials, 1974, pp. 50-73.
The vibrations produced by the inertial load are presented in FIG. 1, which also gives schematic presentation of the events in the beginning of the impact.
At the moment marked 0, the tup touches the specimen and starts to accelerate it. At the moment 1 the midsection of the specimen has reached the speed of the hammer and the force reaches its first peak. Because the specimen and the tup are compressed during the acceleration, the specimen tries to fly off and the force diminishes after the moment 1. At the same time, the specimen starts to bend and the reaction forces start to appear at the supports. The bending of the specimen consumes rapidly the extra kinetic energy of the specimen and the tup hits the specimen a second time beginning at the moment 2. Because the speed differences are smaller this time, the force generated by the acceleration is smaller. The same process of hitting and getting loose then repeats itself several times with decreasing force-amplitude and the force is summed onto the actual bending force.
A very harmful effect of the inertial load acting on the notched section of the specimen is its suggested interference with the fracture initiation process at the notch root. The high inertial load peak produces a complex stress wave across the notch and according to Seifert et al. (Seifert, K. and Meyer, L. W.: "Moglichkeiten zum Vermindern des Aufschlagimpulses bei Bruchzahigkeitsprufungen unter schlagartiger Beanspruchung", Materialpruf. 19, Nr. 6, Juni 1977) this effect might cause about 20 percent decrease in the fracture load. The same investigators refer also to unpublished finite-element calculations which predict the same effect. These results may partly explain why correlations between the Charpy-V impact energy and other toughness measurements are relatively poor.
Because it is evident that the force measured from the tup does not give reliable results during the first moments of the impact, several methods have been proposed to avoid inertial load effects. One solution is to measure the bending moment directly from the specimen by instrumenting it with strain gauges. This method is time consuming and expensive and it is not applicable to low or high temperature tests. It has also been proposed to calculate the bending force from the elapsed time before specimen breaks. This indirect method is based on the findings that the bending force indirect method is based on the findings that the bending force depends almost linearly of time. The drawback in this method is that it requires the moment when the specimen breaks to be measured very accurately and this can be done only by instrumenting the specimen. Most widely used method is to reduce the impact speed. The lower speed produces less vibrations and makes the analysis of the force-time diagram easier. On the other hand, one loses some of the dynamic nature of the test. The available energy decreases and therefore also the velocity decreases more during the specimen bending and this may increase the scatter of the test results.
It is also possible to move the inertial load effects away from the tup by attaching the specimen to the moving hammer. This method has been found effective by several investigators. In FIG. 2 is presented an actual force-displacement diagram for aluminum Charpy-V specimen (solid line). The other superimposed diagram (dotted line) in FIG. 2 was gained with same kind of specimen but, this time the specimen was tied onto the hammer. The first inertial peak was avoided and the vibrations also became smaller. The explanation for this is that the inertial load is acting on the two supports and because it is now divided on both of the supports, its magnitude is halved and it also takes time for the diminished vibrations to reach the load measuring tup. This method is the only one which also eliminates the harmful transverse stress wave across the notch and is therefore bound to give more reliable results. Unfortunately it is practically impossible to use this method at any other than room temperature, because the attaching of the specimen is time consuming and even if it were possible to attach a very cold or hot specimens in this way, the temperature of the specimen cannot be known at the moment of the actual impact.